Field of the Invention
This invention relates to a novel red monoazo lake pigment which is excellent in various properties including brightness, color strength, gloss, fastness to water and fastness to heat and highly useful in, for example, printing inks, paints, plastics and stationery products.
Description of the Prior Art
A conventional monoazo lake pigment of the following formula (I): ##STR2## may be prepared by, for example, coupling a diazo component, which is obtained by diazotizing an amine of the following formula (II): ##STR3## at a temperature of 5.degree. C. or below, with a coupler component, which is obtained by dissolving an acid of the following formula (III): ##STR4## in an aqueous solution of a caustic alkali metal, at a temperature of 5.degree. C. or below to thereby give a monoazo dye of the following formula (IV): ##STR5## wherein M.sub.2.sup.+ represents a sodium, potassium or lithium ion; and then laking the resulting monoazo dye (IV) by adding an aqueous solution of a strontium salt to a suspension of said monoazo dye (IV) and then allowing the obtained mixture to react at a temperature of 5.degree. C. or below.
An X-ray diffraction pattern of the conventional monoazo lake pigment of the formula (I) thus obtained shows a high diffraction intensity at a diffraction angle (2.theta..+-.0.2.degree.; Cu-K.sub..alpha.) of 5.0; moderate diffraction intensities at 6.4.degree. and 26.4.degree.; and relatively low diffraction intensities at 7.8.degree., 13.9.degree., 16.5.degree., 21.1.degree. and 28.5.degree..
However the monoazo lake pigment having a crystal form of the above mentioned X-ray diffraction pattern, which will be referred to as .alpha. form hereinafter, has not been marketed yet. This is because it is inferior to a presently marketed calcium lake pigment known as C.I. Pigment Red 57:1 in brightness, color strength and gloss when used in a printing ink or a paint; and in fastness to heat when used in coloring plastics, although the former is superior to the latter in fastness to water.